server: fix data race (#536)
server: fix data race (#536)

* server: fix data race

This fixes the detected race.

Fixes #534

* Remove the listener and packetconn from Server

There does not seem a need to store the listener and packetconn again
in the Server structure. The dns.Servers already has access to them
and can also shutdown the handlers.

coredns/coredns,johnbelamaric/coredns,chrisohaver/coredns,jremond/coredns,varyoo/coredns,ii/coredns,johnbelamaric/coredns,ii/coredns,miekg/coredns,miekg/coredns,jremond/coredns,yongtang/coredns